This invention relates to a speaker verification system.
A speaker verification system determines whether or not a speaker is the registered person by comparing a password speech uttered by the speaker with a reference password speech that has been registered in advance. Such a system is useful as identification-verifying means instead of signature in a bank business or as check-in and check-out control systems, and its practical realization has been expected.
In the conventional speaker verification system, the password speech pattern uttered by a registered speaker is stored as a reference pattern, and at the time of verification, a code specifying the speaker (hereinafter called simply the "registered speech number") and the password spoken by a speaker to be verified are inputted. The reference pattern specified by the registered speaker number and the uttered speech pattern of the password (hereinafter called the "input pattern") are compared with each other to calculate the distance therebetween (an evaluation value of dissimilarity between the patterns). If this distance is smaller than a predetermined threshold value, the speaker is judged to be the registered person, and if the distance is greater, on the contrary, the speaker is judged to be an imposter.
The problem with such a speaker verification system lies in the difficulty in the determination of the threshold value. In other words, even when the same person utters a predetermined password, the uttered password pattern varies from time to time, and the degree of this change varies from person to person. Therefore, if the threshold value is set to be large enough to solve this problem, an imposter is frequently misjudged to be the registered person, whereas if the threshold value is set to be smaller, the registered person may be rejected as an imposter.
U.S. Pat. No. 4,581,755 and U.S. Pat. No. 4,694,493 disclose a system which solves the problem of the conventional speaker verification system described above. In accordance with this prior art system, not only the reference pattern of a registered person (hereinafter called the "verification reference pattern") is registered in advance but also the speech pattern of other person which is similar to the verification reference pattern is registered as a control reference pattern are registered in advance. When he input pattern satisfies the following conditions (A) and (B) on the distance between the patterns, the speaker is judged to be registered person and a coincidence signal is generated:
(A) the distance between the input pattern and the verification reference pattern is less than the distance between the input pattern and the control reference pattern; and
(B) the distance between the input pattern and the verification reference pattern is less than a predetermined threshold value.
According to this system, the threshold value to be used in the condition (B) can b set more loosely, and the possibility of misjudgement of the true registered speakers as impostors can be reduced. Moreover, the false acceptance of impostors as true registered speakers can be prevented, too, by the condition (A).
In this prior art system described above, patterns of other persons similar to (having small distance from) the verification reference pattern of the password are selected as the control reference patterns. As a result, the patterns of a large number of speakers must be prepared to provide the patterns of other persons, and when the password varies for each registered person, the patterns of a large number of speakers must be prepared for each of these different passwords. If the system is applied to verify an unlimited number of persons, therefore, it has been very difficult to obtain the control reference patterns.